A well-known type of network is the Public Switched Telephone Network (“PSTN”). The PSTN was originally intended to carry voice communications over telephone lines, however, they are increasingly used to carry electronic data, such as facsimile transmissions and/or modem communications. The internet is another network that is gaining enormous popularity. The internet has typically been used to carry electronic data. However, as bandwidths increase and digitization (such as MPEG) improves, the Internet will increasingly be used to carry voice and video communications. In general, it can be seen that known networks are converging. For example, the distinction between PSTN and Internet will become less meaningful as a single type of network becomes capable of carrying voice, fax, video, data and other forms of electronic communications.
It is also known that networks have failure-modes, or, from the user's perspective, all networks have a certain amount of reliability. It is known to measure network reliability in terms of percentage of time that the network is available (“Availability Measurement”). A common expression of availability measurement is “Five-Nines Availability”, which generally means that the network is available 99.999% of the time, as averaged over a specified period of time. An availability measurement can be used in a variety of ways. For example, network customers purchasing or leasing networks may specify their required availability measurement, and in turn, network designers may use this availability measurement to design and test the network to ensure that it meets the needs of the network customer.
Availability measurements can be useful in designing and or measuring PSTN's that are used for carrying non-critical voice telephone calls, because a user simply needs to know that there is a dial-tone and thus that the user can make a voice telephone call. Accordingly, where the PSTN has “Five-Nines Availability”, then a user can expect a dial-tone to be available 99.999% of the time.
However, an availability measurement may not be useful in designing or measuring networks transporting critical applications. For example, a “Five-Nines Availability” may not be acceptable for voice access, because the availability measurement does not reflect other measurements such as failure frequency, failure duration and failure impact:—all critical attributes to users of networks. (As is known to those of skill in the art, such measurements can also be referred to as “metrics” or “parameters”). If, for instance, the “Five-Nines” is five, one-minute subscriber access outages in a year, then user-downtime requirements and cut-off calls requirements are generally met. If, however, the “Five-Nines” is one-hundred-and-fifty, two-second outages, then the subscriber outage requirements are met but ineffective call-attempt requirements and dropped-call requirements are generally not met. Continuing with the example, if the “Five-Nines” is one, thirty-minute outage every six years that causes a 30,000 subscriber outage then the user-downtime requirements are not met and the network owner needs to submit a report to the FCC that outlines the corrective and preventive action.
The foregoing example shows how an availability measurement can be unhelpful in designing PSTNs, but it will now be apparent to those of skill in the art that such availability measurements are even further ineffectual in the design, monitoring and optimization of more modem, multi-service networks that carry fax, data, voice, video, audio and other forms of electronic communications. It will be further apparent that the foregoing problems are exacerbated where services are delivered to the user across multiple networks controlled by various different network providers.